1. Field of the Invention
The invention relates to a process and an apparatus for manufacturing low-gas and pore-free aluminum casting alloys. Crucible or tank-type furnaces generally are used to make casting alloys. Either liquid electrolysis metal is loaded or solid metal is smelted. The desired alloy composition is adjusted by adding alloy components such as silicon, magnesium, copper, titanium and/or nickel. The smelting bath is heated to dissolve and alloy the components. Increased amounts of hydrogen are absorbed because aluminum in the liquid state has a high dissolving capacity for hydrogen. The latter is produced by the reaction of liquid aluminum with steam and is immediately absorbed atomically by the melt. The steam comes in contact with the molten aluminum through the materials used, the jackets of the oven and crucible, the tools, the melting auxiliaries and flux, the combustion of gaseous and liquid fuels, and the atmospheric humidity. The amount of hydrogen dissolved depends on the temperature of the metal, the composition of the alloy, and the partial pressure of the hydrogen. The hydrogen uptake is favored by open burner flames or vigorous bath movements in induction furnaces. In the refining of casting alloys with alkali and alkaline earth metals such as strontium, sodium and calcium, the hydrogen content of the melt increases considerably to values of more than 0.3 ml hydrogen per 100 g of metal, since steam decomposes even more rapidly under the influence of these metals. The melt should be purified immediately before pouring if possible, since treatment performed too early can lead to contamination once again during subsequent technological steps, for example pouring to transport the melt. In particular, the melt coming in contact with the humidity in the atmosphere results in an increase in hydrogen content and the resultant undesirable increased porosity of the aluminum castings. Usual purification processes are performed with inert as well as chemically-active gases. During flushing with inert gases (argon or nitrogen, for example), the hydrogen is practically physically removed by lowering its partial pressure. This type of hydrogen removal is expensive from the technical standpoint and poses the risk of hydrogen coming in contact with the melt during treatment. In addition, undesirable nitride formation can occur when nitrogen is used with certain alloy components. When chemically active chlorine gas is employed, aluminum chloride is formed and rises to the surface; it produces effective flushing because of its distribution in the melt. Chlorine gas is a serious environmental poison, however, and is also expensive to manufacture. The protective measures required to prevent the escape of the poisonous gas and its reaction products require considerable investment. In contrast to the use of chemical agents, vacuum degasification of the melt is especially environmentally friendly and effective method. However, this method is not optimally successful, mainly because of the costly transportation of the melt, intermediate cooling and remelting after the necessary alloying, refining, and vacuum degasification processes, until continuous casting takes place and the necessary coming into contact with the atmospheric humidity which that involves, so that the alloying and refining process followed by continuous casting does not produce gas-poor and pore-free aluminum casting alloys.